1. Field of the Invention
The invention relates to roller grinding mills.
2. Description of the Background Art
Roller grinding mills have been known for more than a hundred years, and are used throughout the world. They exist in an extremely wide variety of designs. Thus, for example, DE 153 958 C from 1902 shows a cone mill with a revolving grinding plate on which rest eight grinding cones under spring pressure.
Modern mills use grinding rollers that have heavy weights and large diameters to achieve high milling output. Please see DE 198 26 324 C, DE 196 03 655 A, which corresponds to U.S. Pat. No. 6,021,968, and also EP 0 406 644 B. This type of roller grinding mill has gained extremely wide acceptance in practice because it has considerable advantages with regard to design, control, and energy economy. The chief areas of application for modern roller grinding mills are the cement industry and coal-fired power plants. In the cement industry, roller grinding mills are used for producing raw cement meal as well as for clinker grinding and coal grinding. In combination with rotary kilns and calcining installations, the furnace exhaust gases from the heat exchanger and clinker cooler can be used to dry the grinding stock and pneumatically transport the ground stock. In power plants, the roller grinding mills are used to finely grind the coal and feed it directly into the boiler with the aid of the classifier air, if possible without the use of an intermediate bunker.
Modern large mills require drive power levels of up to 10 MW. It is a matter of course that the associated bearings and drives, in particular the transmissions, must be of special design. The teeth, the shaft bearings, the integrated axial thrust bearings and their supports within the transmission housing, are particularly heavily loaded. For drive power levels up to 6 MW, planetary bevel gear transmissions, which are matched to the circular grinding plate on account of their circular shape, have become established as the state of the art; they transmit the static and dynamic grinding forces to the foundation. Please see DE 35 07 913 A or DE 37 12 562 C, which corresponds to U.S. Pat. No. 4,887,489. Pivoted-pad bearings with hydrodynamic and/or hydrostatic lubrication are used as axial thrust bearings; please see DE 33 20 037 C.
These designs, space-saving in and of themselves, have significant disadvantages, however. As soon as a problem arises with just one component, the entire drive must be dismantled. It has proven to be particularly disadvantageous in this regard that it is extremely difficult to visually inspect the gears of the planetary transmission; oftentimes, this is not possible until the drive has been dismantled completely. Since these drives are special designs, procurement of replacement parts takes a commensurately long time, i.e., weeks or months, since stocking of replacement parts is considered too cost-intensive on account of the special designs. This is unsatisfactory.
Another disadvantage of the prior art drive design is what is called the maintenance drive, which rotates the grinding plate during certain maintenance and repair operations, but which only functions as long as the primary transmission itself functions.
Naturally, there has been no shortage of proposals for doing away with these inadequacies and disadvantages. Thus, DE 39 31 116 C shows a drive device for a roller grinding mill having a grinding plate that can rotate about a vertical axis, which has a crown gear connected to the lower part of the grinding plate. Moreover, two diagonally arranged drives are provided, each having a drive motor and a gear reducer. Each gear reducer has two pinions that mesh with the crown gear of the grinding plate.
Known from DE 76 29 223 U is a roller grinding mill with a ring gear located under its grinding plate. The pinions of four hydraulic motors fastened to the base of the mill housing mesh with the ring gear.
Despite the theoretical advantages of these multiple-motor drive concepts, they have as yet been unable to gain acceptance in practice. In the case of hydraulic drives, the lower efficiency as compared with electric drives, and the lower availability and service life of the hydraulic components are disadvantages. The previously described dual-drive concept with electric motors and gear reducer was unable to gain acceptance because considerable excess torques arise during operation, which can result in overloading of the transmission to the point of destruction. Moreover, it was not possible to support mill operation with the required capacity in the event of the failure of a drive.
However, it is not only the required drive power level that has increased with the increasing capacity of roller grinding mills, but also the number of grinding rollers rolling on the grinding plate. Thus, DE 103 43 218 B4, which corresponds to U.S. Publication NO. 20080245907, describes a roller grinding mill with six grinding rollers and a single drive. Here, the design is arranged such that two diagonally opposite grinding rollers can be pivoted out simultaneously, and the mill is intended to produce 80% of the full milling output with the remaining four active grinding rollers. A disadvantage in this design is that two grinding rollers always have to be pivoted out, even when only one grinding roller has failed.
DE-OS 21 24 521 has also already described a roller grinding mill with six grinding rollers.
Finally, a roller grinding mill with four grinding rollers is known from DE 197 02 854 A1, in which each grinding roller is driven by a separate drive, having an electric motor and gear reducer. The grinding plate itself does not have a drive. No provision is made for deactivation of one or more grinding rollers or of one or more drives.